Electrical Machines Questions and Answers – Machine Ratings

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This set of Electrical Machines Multiple Choice Questions & Answers (MCQs) focuses on “Machine Ratings”.

1. Refer the diagram; a single-phase supply of 100V, 50Hz is fed to the step-up transformer having primary impedance of 1 ohms and secondary impedance 20 ohms. What is the power consumed by the load of 5 ohms?
The power consumed by load of 5 ohms is 20 W of 1 ohms & secondary impedance 20 ohms
a) 20 W
b) 40 W
c) 10 W
d) 100 W
View Answer

Answer: a
Explanation: The referred resistance to the secondary, Z2=1*(52) = 25 ohms
Total resistance at secondary = 25+20 = 45 ohms
Total resistance seen by source = 45+5 = 50 ohms
Hence, current in secondary = 100/50 = 2 A
Power consumed by the load = I2 *R = 4*5 = 20 W.
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2. If the dimension of a single phase transformer is made sqrt(3) times less than the original structure, then what is new no load current rating of the transformer?
a) 1/sqrt(3)
b) sqrt(3)
c) 3
d) 1/3*sqrt(3)
View Answer

Answer: a
Explanation: The no load current of the transformer is directly proportional to the dimension of the transformer.

3. For a single phase, 50 Hz transformer, if the open circuit test is conducted at 40 Hz. What is the variation in the no-load power factor of the transformer?
a) Decreases
b) Increases
c) Remains constant
d) None of the mentioned
View Answer

Answer: a
Explanation: If the frequency reduces then flux density increases, so the flux and then the magnetizing current. When magnetizing current increases power factor angle increases so the power factor decreases.

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4. The desired voltage regulation for a single phase, 50 Hz transformer is zero. Its pu ohmic drop is 4% and pu reactive drop is 4%. The required power factor angle for the operation in degrees is?
a) 45
b) 90
c) 30
d) 60
View Answer

Answer: a
Explanation: The power factor angle is, tan(phi)=pu resistive drop/pu reactive drop
= 4/4 = 1.

5. While conducting lab experiments on DC machine loading, the students are given a machine with its rating as 100 kW maximum rating. Then the _____________
a) machine can’t be overloaded after 100 kW
b) machine can be overloaded after 100 kW
c) machine can be overloaded by 5% for a small time duration
d) machine can be overloaded by 10%
View Answer

Answer: a
Explanation: As the maximum rating is specified and operation condition are also same, it cannot be overloaded beyond the maximum.

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6. A 3-phase, five limbed shell type transformer, star connected, 200V, 50 Hz, 100 kVA will have ___________
a) no third harmonic current in its phase but has sinusoidal flux
b) third harmonic current in its phase but has flat-topped flux
c) no third harmonic current in its phase but has flat-topped flux
d) third harmonic current in its phase but has sinusoidal flux
View Answer

Answer: a
Explanation: Star connection in three phase has no closed path for the flow of the third harmonic current.

7. For a separately excited dc machine of 25 kW, 250 V and armature resistance of 0.25 ohms, is running at 3000 rpm with supply of 255 V. The electromagnetic power produced at the armature in kilo watts is?
a) 5
b) 10
c) 0.5
d) 8
View Answer

Answer: a
Explanation: The current in armature, Ia = 255-250/0.25 = 20 A
Power = Eb*Ia = 250*20 = 5000 W.
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8. If two alternators A and B are operated in parallel having per unit synchronous reactance as 0.02 ohms and 0.05 ohms respectively.Then what the ratio of contributing loading of machine A to B?
a) 2.5
b) 0.4
c) 1
d) can not be operated in parallel
View Answer

Answer: a
Explanation: The load shared by the alternators is inversely related to their pu reactance.

9. A 200V, 10 hp, 4 poles, 60 Hz, Y-connected induction motor had full load slip of 5%. Then the rotor speed of the motor with respect to stator is?
a) 1710 rpm
b) 1800 rpm
c) 90 rpm
d) 3510 rpm
View Answer

Answer: a
Explanation: Synchronous speed, Ns = 120*f/P = 120*60/4 = 1800 rpm
Rotor speed = (1-slip)*Ns = 0.95*1800 = 1710 rpm.
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10. Auto transformer rating is chosen so as to have best performance when the analogous two winding transformer has ___________
a) two voltage levels fairly close to each other
b) very high transformation ratio
c) any voltage levels
d) step down purpose
View Answer

Answer: a
Explanation: The auto transformer is best suited with the voltage levels of same rating for a two winding transformer.

11. In the lab, two students X and Y conducted the experiments to verify the name plate ratings of the transformer.But student X conducted the same at higher frequency. The kVA reading of student X will be ___________
a) higher than Y’s reading
b) lower than Y’s reading
c) same as Y’s reading
d) none of the mentioned
View Answer

Answer: a
Explanation: As rating is directly proportional to the frequency of operation.

Sanfoundry Global Education & Learning Series – Electrical Machines.

To practice all areas of Electrical Machines, here is complete set of 1000+ Multiple Choice Questions and Answers.

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Manish Bhojasia, a technology veteran with 20+ years @ Cisco & Wipro, is Founder and CTO at Sanfoundry. He lives in Bangalore, and focuses on development of Linux Kernel, SAN Technologies, Advanced C, Data Structures & Alogrithms. Stay connected with him at LinkedIn.

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